Thermal control apparatus

ABSTRACT

Thermal control apparatus for controlling the temperature of a photo-developing bath including electrically powered filaments disposed in the bath and energized by a control circuit responsive to the temperature of the bath. The control circuit includes a voltage divider connected across two leads connected to a conventional house outlet having the control point thereof connected to a four layer diode. The output signal from the diode is connected to gate the first of two opposite polarity silicon controlled rectifiers, with the second rectifier being capacitorconnected to gate at the end of the pulse of the first rectifier. The upper leg of the voltage divider includes a positive thermal coefficient resistor thermally coupled to the first rectifier and the lower leg includes a thermistor thermally responsive to the temperature of the bath.

United States Patent 1191 Kuzyk Dec. 18, 1973 THERMAL CONTROL APPARATUSPrimary Examiner-Bernard A Gilheany Assistant ExaminerF. E. Belu I R K k4 A [76] nvemor 1221? 36 Schiller ve Att0rney-l. Michael Bak-Boychuk etal.

[22] Filed: May 10, 1972 57 ABSTRACT 21 A l N 252,079 Thermal controlapparatus for controlling the temperature of a photo-developing bathincluding electrically powered filaments disposed in the bath andenergized {52] US. Cl 219/501, 219/497, 2l9/505 by a-comrol Circuitresponsive to the temperature of [5 lift. Cl. the bath. The controlcircuit includes a voltage divider [58] Field of Search 219/327, 3238,4497, connected across two leads connected to a com/em 219/499 86 tionalhouse outlet having the control point thereof connected to a four layerdiode. The output signal [56] References cued from the diode isconnected to gate the first of two op- UNlTED STATES PATENTS positepolarity silicon controlled rectifiers, with the 3,431,399 3/ 1969Venning 219/497 second rectifier being capacitor-connected to gate at3,586,829 6/1971 Farmer et al. 219/497 the end of the pulse of the firstrectifier. The upper leg 3,465,123 9/1969 HBITiS 219/328 of the voltagedivider includes a positive thermal 00 i: 3,400,246 9/1968 Zob 219/328ficient resistor thermally coupled to the first rectifier 523 2: E i 54:8 and the lower leg includes a thermistor thermally re- 370s720 1/1973Whitney ;ii'.f.'.' 1. .7 317 41 Sponsive to the temperature of the bath'8 Claims, 4 Drawing Figures THERMAL CONTROL APPARATUS BACKGROUND OF THEINVENTION The present invention relates to thermal control apparatus andmore particularly to electrical control apparatus for controlling thetemperature of a photodeveloping bath.

In the art of chemical developing of photo-sensitive paper or negativesthe quality and clarity of the reproduction is typically related to thechemical accuracy of the developing bath and the accuracy of thetemperature thereof. Particularly in the developing of color prints thecontrol over the temperature of the developing bath is critical and manyautomatically controlled heating devices have been developed in the pastto maintain the bath at a preselected constant temperature. Mostfrequently such devices included electrical heating filaments powered byconventional house current and controlled by various arrangements ofthermally sensitive circuits, including devices like thermistors,wherein hardover 'control was utilized, switching in and out the powerto the filament according to the temperature sensed by the controlcircuit. These techniques often resulted in control errors due to thethermal inertia of the filaments, the control time constants of thesystem and the volume or heat inertia of the bath, often resulting in anexcessive over and under compensation and therefore producing arelatively inaccurate thermal environment for the chemical reaction.Further improvements in the prior art for reducing the above-describederrors typically involved extensive circuitry, with the attendant costand maintenance requirements, precluding convenient utility thereof forhome use or use by small development enterprises.

SUMMARY OF THE INVENTION Accordingly, it is the general purpose andobject of the present invention to provide a relatively simplephoto-developing bath heater which offers the desired thermal accuracyfor high fidelity photo developing.

Other objects of the invention are to provide a photo bath heater whichis adapted to conventional home power sources and which also isrelatively free of electrical noise.

Briefly these and other objects are accomplished within the presentinvention by providing a voltage divider connected across two leadsextending from a conventional power outlet, where the voltage dividerincludes a positive thermal coefficient resistor on the upper legthereof an an adjustable potentiometer connected in series with anegative temperature coefficient thermistor in the lower leg thereof.The thermistor is attached for intimate thermal contact with a photobath container, registering the temperature thereof in the form ofchanges in resistance, according to conventional operation ofthermistors. Disposed within the container are a plurality of heaterfilaments, exposed to intimate thermal contact with the developing bath,for transferring heat thereto. The control point, or juncture betweenthe upper and lower legs of the voltage divider is connected to theinput side of a four layer diode, generally referred to as a Shockleydiode or Silicon Unilateral Switch, which at the output thereof isconnected to the gate terminal of a first silicon controlled rectifier.The first rectifier connects the heater filaments with the return leadfrom the power outlet, conducting when the gate terminal is energized. Asec- 0nd silicon controlled rectifier, reversed in polarity to conductthe opposite half of the current cycle, is capacitively coupled to thefirst rectifier to conduct the opposite half of the cycle after thefirst rectifier cuts off. The first silicon controlled rectifier isfurther thermally coupled with the positive temperature coefficientresistor in the upper leg of the voltage divider whereby the controlpoint triggering the four layer diode is cycled by the temperatureoutput of the first SCR, increasing the proportionality of the control.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a firstembodiment of a thermistor control switching circuit constructedaccording to the present invention;

FIG. 2 is an illustration of the mechanical arrangement for thermalconnection between R2 and SCRI of FIG. 1;

FIG. 3 is a graphical illustration of the electrical and thermalfunctions at selected points of FIG. 1; and

FIG. 4 is yet another embodiment of the structure shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1 a heatercircuit 10 is connected by leads 13 and 14 to one side of a conventionaltwo lead receptacle 15 which is connected to a typical home power sourceE by corresponding leads 11 and 12 extending from the other sidethereof. A resistive voltage divider generally designated 25 isconnected across leads l3 and 14, comprising a first resistor Rconnected at one end to lead 13, a second positive thermal coefficientresistor R connected at the one end thereof to the other end of resistorR and an adjustable potentiometer R connected at one end to resistor Rand at the other end to a conventional negative temperature coefficientthermistor T. Thermistor T in turn connects at the other end thereof tothe return lead l4 completing the shunt connection of divider 25.Thermistor T is furthermore attached for intimate thermal contact to aphoto-bath container 20, responding to the temperature thereof.

The connection between resistor R and potentiometer R also serves as thecontrol or division point 0 of divider 25, being also connected to theinput terminal of a four layer or Shockley diode D Diode D,characteristically acts as a switching diode changing to a conductivestate upon receiving a signal above a preselected voltage level andremaining in the conductive state until the input signal goes to a zeropotential, at

which time diode D resets for the next switching con dition. The outputterminal of diode D is connected to the gate terminal of a first siliconcontrolled rectifier SCR gating the rectifier to conduct following theswitching of diode D until the voltage across rectifier SCR returns tozero. Rectifier SCR connects at the input side thereof to respective oneends of heater filaments R and R extending through photo-bath container20, which at the other ends thereof connect to lead 13. Thus, whenrectifier SCRQ conducts the filaments R and R are energized, giving offheat and thereby warming any fluid withn the container.

In order to further increase the heating capacity of the filaments thegate terminal of a second silicon controlled rectifier SCR is capacitorcoupled with the input terminal of the first rectifier SCR,.Specifically a capacitor C is connected at one end to the input terminalof rectifier SCR, and at the other end to the gate terminal of rectifierSCR and to one end of a resistor R Resistor R, is connected at the otherend of lead 13, forming a delay circuit in combination with capacitor Cfor triggering rectifier SCR after rectifier SCR turns off. RectifierSCR is connected to conduct the negative side of the cycle, therebyconducting current through filaments R and R for a duration in eachcycle determined by the delay of capacitor C,

As shown in FIG. 2 resistor R of the upper leg of voltage divider 25 ismounted in common with the first rectifier SCR on a heat sink l8abutting the underside of a circuit board 17 containing circuit 10.Accordingly, each time rectifier SCR is gated to conduct the heat outputthereof is conducted by heat sink 18 to resistor R Resistor R is apositive temperature coefficient resistor, increasing in resistance withan increase in temperature and raising the resistance in the upper legof divider 25 to lower the voltage at the control point as thetemperature of rectifier SCR1 increases. In this manner the voltage atthe control point 0is lowered until it falls below the triggering levelof diode D,, at which time rectifier SCR, no longer ature of resistor RfThus a secondary loop is formed, thermally pulse modulated, allowingfor a more proportioned control as further described herein below.

The description of the operation of the present invention will now beset forth with reference to FlGS. l and 2 and with particular referenceto FIG. 3.

As shown in FIG. 3 the source E generates a sinusoidal signal A, such asis typically available in a conventional wall outlet, of an amplitude ofl 15 to 120 volts at 60 cycles per second. Voltage divider 25 dividesthe source voltage A at control point 0 to form a voltage signal B at anamplitude determined by the ratio of the sum of resistances of resistorsR and R and the sum of the resistances of potentiometer R and thermistorT. A voltage level C, corresponding to the triggering voltage of diode Dis shown referenced against signal B whereby each time voltage B exceedsvoltage C diode D, is triggered, gating rectifier SCR until voltage Bcrosses zero. Rectifier SCR,, in turn, heats up resistor R along atemperature curve H, raising the resistance R, and R of the upper leg ofthe divider 25 and dropping voltage B. As voltage B drops the on-time ofSCR,

1 decreases to turn off the power applied to filaments R As shown inFIG. 4 yet another embodiment of the' circuit includes a voltage divider125 having a capacitor C in the upper leg thereof. Capacitor Ceffectively phase-shifts the voltage at the control point of divider 125to trigger rectifier SCR substantially proximate the positive zerocrossing of signal E. ln this manner the switching noise of rectifierSCR is reduced to a minimum without modifying the basic operationthereof. Furthermore a Zener diode Z is connected across resistor Rpotentiometer R and thermistor T of voltage divider 125, stabilizing anyvariations in line voltage or voltage signal E.

Some of the many advantages of the present invention should now bereadily apparent. The invention allows for a relatively large heattransfer capacity through the heating filaments, with the associatedheat inertia, at the same time limiting the loop errors thereof. Thus,the initial thermal errors are quickly compensated and the steady stateoperation of the system is controlled to small overshoots resulting in aconstant temperature of the bath.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. Accordingly, theinvention may be practiced otherwise than as described within the scopeof the appended claims.

What is claimed is:

l. A thermal control system for controlling the temperature of a fluidin a container, comprising, in combination:

container means adapted to receive the fluid;

electrical heating means disposed within said container means fortransferring heat to the fluid upon passing electrical currenttherethrough; and power supply means connected to said electricalheating means including resistive means responsive both to thetemperature of the fluid and the internal temperature of said powersupply means, gating means operatively connected for switching to saidresistive means for conducting a series of continually decreasing pulsesof electrical current to said heating means from said power supply meanscontrolled by said gating means to provide a first pulse of a widthfunctionally dependent on the temperature difference between apreselected temperature and the temperature of the fluid and the otherconsecutive pulses being further functionally dependent in width on theinternal temperature of said 7 gating means. I

2. A temperature control system according to claim 1 further comprising:

said electrical heating means including a plurality of resistivefilaments connected to receive at respective one ends thereof saidelectrical current and connected at th other ends thereof to return saidcurrent.

3. A temperature control system according to claim 2, furthercomprising:

said power supply means including a source of alternating electricalpower, a voltage divider having a resistive upper leg temperatureresponsive by an increase in resistance with an increase in thetemperature, a resistive lower leg including an adjust ablepotentiometer and a negative temperature coefficient thermistor disposedto sense the temperature of said container means, said upper leg beingconnected at one end thereof to one side of said power source and saidlower leg connecting at the one end thereof to the other side of saidpower source, the respective other ends of said legs being connected incommon, a four layer diode connected at the input terminal thereof tothe connection between said upper and lower legs and rectifier meansconnected to receive the output signal from said diode and disposed toconduct the thermal output thereof to the upper leg of said voltagedivider.

4. A thermal control system according to Claim 3,

further comprising:

said rectifier means including a first silicon controlled rectifierconnected between the other side of said power source and the other endsof said filaments, at second silicon controlled rectifier connected inopposing polarity therewith, said first rectifier being connected at thegate terminal thereof to the output terminal of said diode, and saidsecond rectifier being capacitor coupled at the gate terminal thereofwith said first rectifier.

therethrough; and

power supply means connected to said electrical heating means includingresistive means responsive both to the temperature of the fluid and theinternal temperature of said power supply means, gating meansoperatively connected for switching to said resistive means forconducting a series of continually decreasing pulses of electricalcurrent to said heating means from said power supply means con- Athermzihcomml System according to claim trolled by said gating means toprovide a first pulse further of a width functionally dependent on thetempera i voliage qmdcr mcludmg a capacltor comtected ture differencebetween a preselected temperature "i sdenes with fupper q i a z and thetemperature of the fluid and the other con- 3:0; igi t i t ig ser a n dlo ij er lzjg s m para le secutive pulses being further functionallydependent in width on the internal temperature of said 6. A thermalcontrol system according to claim 5 further comprising:

said rectifier means including a first silicon congating means.

8. A thermal control apparatus according th claim 7, further comprising:

perature of matter, comprising, in combination:

electrical heating means adapted to be disposed for thermal contact withthe matter for transferring heat thereto upon having electrical currentpassed said power supply means including a source of alternatingelectrical power, first means connected to receive said electrical powerand disposed to sense the temperature of the matter for passing selectedpulses of said power to said heating means of a pulse widthcorresponding to the difference between a preselected temperature andand sensed temperature, and second means operatively connected to saidfirst means for decreasing said pulses upon an increase in thetemperature of said first means.

1. A thermal control system for controlling the temperature of a fluidin a container, comprising, in combination: container means adapted toreceive the fluid; electrical heating means disposed within saidcontainer means for transferring heat to the fluid upon passingelectrical current therethrough; and power supply means connected tosaid electrical heating means including resistive means responsive bothto the temperature of the fluid and the internal temperature of saidpower supply means, gating means operatively connected for switching tosaid resistive means for conducting a series of continually decreasingpulses of electrical current to said heating means from said powersupply means controlled by said gating means to provide a first pulse ofa width functionally dependent on the temperature difference between apreselected temperature and the temperature of the fluid and the otherconsecutive pulses being further functionally dependent in width on theinternal temperature of said gating means.
 2. A temperature controlsystem according to claim 1 further comprising: said electrical heatingmeans including a plurality of resistive filaments connected to receiveat respective one ends thereof said electrical current and connected atth other ends thereof to return said current.
 3. A temperature controlsystem according to claim 2, further comprising: said power supply meansincluding a source of alternating electrical power, a voltage dividerhaving a resistive upper leg temperature responsive by an increase inresistance with an increase in the temperature, a resistive lower legincluding an adjustable potentiometer and a negative temperaturecoefficient thermistor disposed to sense the temperature of saidcontainer means, said upper leg being connected at one end thereof toone side of said power source and said lower leg connecting at the oneend thereof to the other side of said power source, the respective otherends of said legs being connected in common, a four layer diodeconnected at the input terminal thereof to the connection between saidupper and lower legs and rectifier means connected to receive the outputsignal from said diode and disposed to conduct the thermal outputthereof to the upper leg of said voltage divider.
 4. A thermal controlsystem according to Claim 3, further comprising: said rectifier meansincluding a first silicon controlled rectifier connected between theother side of said power source and the other ends of said filaments, asecond silicon controlled rectifier connected in opposing polaritytherewith, said first rectifier being connected at the gate terminalthereof to the output terminal of said diode, and said second rectifierbeing capacitor coupled at the gate terminal thereof with said firstrectifier.
 5. A thermal control system according to claim 3, furthercomprising: said voltage divider including a capacitor connected inseRies with the upper leg thereof and a Zener diode connected from saidcapacitor in parallel across said upper and lower legs.
 6. A thermalcontrol system according to claim 5 further comprising: said rectifiermeans including a first silicon controlled rectifier connected betweenthe other side of said power source and the other ends of saidfilaments, a second silicon controlled rectifier connected in opposingpolarity therewith, said first rectifier being connected at the gateterminal thereof to the output terminal of said diode, and said secondrectifier being capacitor-coupled at the gate terminal thereof with saidfirst rectifier.
 7. A thermal control system for controlling thetemperature of matter, comprising, in combination: electrical heatingmeans adapted to be disposed for thermal contact with the matter fortransferring heat thereto upon having electrical current passedtherethrough; and power supply means connected to said electricalheating means including resistive means responsive both to thetemperature of the fluid and the internal temperature of said powersupply means, gating means operatively connected for switching to saidresistive means for conducting a series of continually decreasing pulsesof electrical current to said heating means from said power supply meanscontrolled by said gating means to provide a first pulse of a widthfunctionally dependent on the temperature difference between apreselected temperature and the temperature of the fluid and the otherconsecutive pulses being further functionally dependent in width on theinternal temperature of said gating means.
 8. A thermal controlapparatus according th claim 7, further comprising: said power supplymeans including a source of alternating electrical power, first meansconnected to receive said electrical power and disposed to sense thetemperature of the matter for passing selected pulses of said power tosaid heating means of a pulse width corresponding to the differencebetween a preselected temperature and and sensed temperature, and secondmeans operatively connected to said first means for decreasing saidpulses upon an increase in the temperature of said first means.